Method of atomic layer etching using hydrogen plasma

ABSTRACT

A method for etching a target layer on a substrate by a dry etching process includes at least one etching cycle, wherein an etching cycle includes: depositing a carbon halide film using reactive species on the target layer on the substrate; and etching the carbon halide film using a plasma of a non-halogen hydrogen-containing etching gas, which plasma alone does not substantially etch the target layer, thereby generating a hydrogen halide as etchant species at a boundary region of the carbon halide film and the target layer, thereby etching a portion of the target layer in the boundary region.

CROSS-REFERENCE TO RELATED APPLICATION

This Application claims priority to U.S. Provisional Patent ApplicationNo. 62/512,991, entitled “METHOD OF ATOMIC LAYER ETCHING USING HYDROGENPLASMA,” filed May 31, 2017, the disclosure of which is herebyincorporated herein by reference.

BACKGROUND Field of the Invention

The present invention generally relates to a method of cyclic dryetching of a layer constituted by silicon or metal oxide, for example.

Description of the Related Art

Atomic layer etching (ALE) is cyclic, atomic layer-level etching usingan etchant gas adsorbed on a target film and reacted with excitedreaction species, as disclosed in Japanese Patent Laid-open PublicationNo. 2013-235912 and No. 2014-522104. As compared with conventionaletching technology, ALE can perform precise, atomic layer-levelcontinuous etching on a sub-nanometer order to form fine, narrowconvex-concave patterns and may be suitable for, e.g., double-patterningprocesses. As an etchant gas, Cl₂, HCl, CHF₃, CH₂F₂, CH₃F, H₂, BCL₃,SiCl₄, Br₂, HBr, NF₃, CF₄, C₂F₆, C₄F₈, SF₆, O₂, SO₂, COS, etc. areknown. However, it is revealed that in-plane uniformity of etching of afilm on a substrate by ALE is not satisfactory when etching an oxidemineral film such as silicon oxide film.

When etching Si or GaAs by ALE using Cl₂ as an etchant gas, relativelygood in-plane uniformity of etching can be obtained. However, whenetching a silicon oxide film by ALE using a fluorocarbon such as C₄F₈ asan etchant gas, good in-plane uniformity of etching is not obtained.This is because the etchant gas is adsorbed on a surface of a substratethrough physical adsorption, not chemical adsorption, despite the factthat conventionally, the adsorption of an etchant gas is sometimescalled “chemisorption.” That is, conventional ALE etches a metal orsilicon oxide film by etchant gas physically adsorbed on its surface,wherein the adsorbed etchant gas reacts with excited species, and alsoby etchant gas which remains in the reaction space after being purged,causing gas-phase etching. As a result, in-plane uniformity of etchingsuffers. If an etchant gas is chemisorbed on a surface of a substrate,the adsorption is “chemisorption” which is chemical saturationadsorption which is a self-limiting adsorption reaction process, whereinthe amount of deposited etchant gas molecules is determined by thenumber of reactive surface sites and is independent of the precursorexposure after saturation, and a supply of the etchant gas is such thatthe reactive surface sites are saturated thereby per cycle (i.e., theetchant gas adsorbed on a surface per cycle has a one-molecule thicknesson principle). When chemisorption of an etchant gas on a substratesurface occurs, high in-plane uniformity of etching can be achieved.Conventional ALE, even though it calls adsorption “chemisorption,” infact adsorbs an etchant gas on a substrate surface (e.g., SiO₂ and SiN)by physical adsorption. If adsorption of an etchant gas ischemisorption, in-plane uniformity of etching should logically be highand also the etch rate per cycle should not be affected by the flow rateof the etchant gas or the duration of a pulse of etchant gas flow afterthe surface is saturated by etchant gas molecules. However, none ofconventional etchant gases satisfies the above.

The above and any other discussion of problems and solutions in relationto the related art has been included in this disclosure solely for thepurpose of providing a context for the present invention, and should notbe taken as an admission that any or all of the discussion was known atthe time the invention was made.

SUMMARY

In some embodiments, a film constituted by components of an etchant(which may be referred to as “an etchant film”) is deposited on asurface of a target layer, and then, the etchant film as well as thetarget layer are etched using plasma treatment. By conducting thedeposition step and the etching step and repeating them alternately asnecessary, the target layer can be etched by a substantially constantpredetermined quantity at each time of conducting the deposition stepand the etching step as an etching cycle. In the above, the etchantcomponents do not serve initially as an etchant gas which etches thetarget layer, but form a film on the surface of the target layer. Inthis disclosure, a “film” refers to a layer continuously extending in adirection perpendicular to a thickness direction substantially withoutpinholes to cover an entire target or concerned surface of the target,and typically a film is formed through reaction using reactive species,rather than simply formed by chemical or physical adsorption of gasmolecules on the surface; thus, the film can grow in a thicknessdirection beyond an atomic layer thickness as a deposition processcontinues. Further, “depositing a film” is not merely providing reactivesites on a target layer surface, but is physically depositing a filmhaving a certain thickness.

In some embodiments, when the etchant film deposited on the surface ofthe target layer is etched by reactive ions such as hydrogen plasma, thecomponents of the etchant film are dissociated and reacted with thereactive ions, generating reactive etchant species which can etch aportion of the target layer at a boundary between the etchant film andthe target layer. In the above, only a certain thickness of the etchantfilm at the boundary can contribute to etching reaction of the targetlayer because the reactive etchant species need to be generated in avicinity of the boundary. Further, since the reactive ions which etchthe etchant film do not etch the target layer, when the etchant film isremoved by the reactive ions, the etching reaction of the target layerstops. Thus, by the above method, a constant amount of the target layercan always be etched by one etching cycle, and thus, controllability andoperability of the etching processes are high.

For example, when the target layer is constituted by SiO₂, afluorocarbon film (CF film) is deposited as an etchant film on a surfaceof the target layer by plasma-enhanced CVD or thermal CVD, followed byexposing the etchant film to a hydrogen plasma, so as to remove theetchant film and simultaneously etch the surface of the target layer. Inthe above, “simultaneously” refers to occurring substantially orpredominantly at the same time or substantially or predominantlyoverlapping timewise. In the above, the depth of the etched portion ofthe target layer increases as the thickness of the etchant filmincreases; however, the depth of the etched portion reaches a plateauand no longer increases when the thickness of the etchant film reaches acertain value. Similarly, the depth of the etched portion of the targetlayer increases as the duration of exposure of the etchant film to theplasma increases; however, the depth of the etched portion reaches aplateau and no longer increases when the duration of exposure of theetchant film to the plasma reaches a certain value. That is, in someembodiments, the above-discussed etching process is a self-limitingreaction process with the two parameters having saturation points.

In the above, the CF film is removed by the hydrogen plasma as gasessuch as CH₄, HF, etc., and in a region in the vicinity of a surface ofthe SiO₂ layer, a portion of the SiO₂ layer also is simultaneouslyremoved as gases such as SiF₄, H₂O, etc. Only a portion of the CF filmnear the boundary contributes to removal of the portion of the SiO₂layer, and the remaining portion of the CF film does not contribute toremoval of the SiO₂ layer, but is simply removed by the hydrogen plasma.Thus, the depth of the etched portion of the SiO₂ layer reaches aplateau in relation to the thickness of the CF film. Also, since etchingof the SiO₂ layer using the hydrogen plasma is effective only when theCF film exists, when the CF film is removed (used up), the etching ofthe SiO₂ layer stops, i.e., the depth of the etched portion of the SiO₂layer reaches a plateau also in relation to the duration of the hydrogenplasma exposure.

In some embodiments, the target layer can be constituted by any material(e.g., SiO₂, TiO₂) which can be etched using a hydrogen halide derivedfrom a CF film. The above etching has high selectivity as to thematerial constituting the target layer, and thus, for example, a filmconstituted by silicon or metal oxide can be selectively etched withoutsubstantially etching a film constituted by silicon or metal nitride orcarbide. In some embodiments, the target layer can be constituted by amaterial which can be etched using a halogen other than fluorine, aslong as a suitable etchant film is selected. Typically, the etchant filmis exposed to an oxygen plasma in a reactive ion etching (RIE) process.Since the etching process involves a self-limiting reaction process (orsaturation process), high controllability can be realized.

For purposes of summarizing aspects of the invention and the advantagesachieved over the related art, certain objects and advantages of theinvention are described in this disclosure. Of course, it is to beunderstood that not necessarily all such objects or advantages may beachieved in accordance with any particular embodiment of the invention.Thus, for example, those skilled in the art will recognize that theinvention may be embodied or carried out in a manner that achieves oroptimizes one advantage or group of advantages as taught herein withoutnecessarily achieving other objects or advantages as may be taught orsuggested herein.

Further aspects, features and advantages of this invention will becomeapparent from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will now be described withreference to the drawings of preferred embodiments which are intended toillustrate and not to limit the invention. The drawings are greatlysimplified for illustrative purposes and are not necessarily to scale.

FIG. 1 is a schematic representation of a plasma-assisted cyclic etchingapparatus usable in an embodiment of the present invention.

FIG. 2 shows a schematic process sequence of plasma-assisted cyclicetching in one cycle according to an embodiment of the present inventionwherein a step illustrated in the sequence represents an ON statewhereas no step illustrated in the sequence represents an OFF state, andthe length of each ON and OFF states does not represent duration of eachprocess.

FIG. 3 illustrates schematic drawings of one cycle of an etching processaccording to an embodiment of the present invention, wherein (a)represents a deposition step, (b) represents a pre-etching step, and (c)represents an etching step.

FIG. 4 is a flow chart showing an atomic layer etching (ALE) accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In this disclosure, “gas” may include vaporized solid and/or liquid andmay be constituted by a single gas or a mixture of gases. In thisdisclosure, a process gas introduced to a reaction chamber fordeposition through a showerhead may be comprised of, consist essentiallyof, or consist of an etchant gas and an additive gas. The additive gastypically includes a dilution gas for diluting the etchant gas andreacting with the etchant gas when in an excited state. The etchant gascan be introduced with a carrier gas such as a noble gas. Also, a gasother than the process gas, i.e., a gas introduced without passingthrough the showerhead, may be used for, e.g., sealing the reactionspace, which includes a seal gas such as a noble gas. In someembodiments, the term “etchant gas” refers generally to at least onegaseous or vaporized compound that participates in etching reaction thatetches a target layer on a substrate, and particularly to at least onecompound that deposits on the target layer in an excited state andetches the target layer when being activated by a plasma. The term“reactant gas” refers to at least one gaseous or vaporized compound thatcontributes to deposition of the etchant film, activation of the etchantfilm, or catalyzes an etching reaction by components of the etchantfilm. The reactant gas can serve as a purging gas. The dilution gasand/or carrier gas can serve as “reactant gas”. The term “carrier gas”refers to an inert or inactive gas in a non-excited state which carriesan etchant gas to the reaction space in a mixed state and enters thereaction space as a mixed gas including the etchant gas.

Further, in this disclosure, any two numbers of a variable canconstitute a workable range of the variable as the workable range can bedetermined based on routine work, and any ranges indicated may includeor exclude the endpoints. Additionally, any values of variablesindicated (regardless of whether they are indicated with “about” or not)may refer to precise values or approximate values and includeequivalents, and may refer to average, median, representative, majority,etc. in some embodiments. Additionally, the terms “constituted by” and“having” refer independently to “typically or broadly comprising”,“comprising”, “consisting essentially of”, or “consisting of” in someembodiments. Further, an article “a” or “an” refers to a species or agenus including multiple species. In this disclosure, any definedmeanings do not necessarily exclude ordinary and customary meanings insome embodiments.

In the present disclosure where conditions and/or structures are notspecified, the skilled artisan in the art can readily provide suchconditions and/or structures, in view of the present disclosure, as amatter of routine experimentation. In all of the disclosed embodiments,any element used in an embodiment can be replaced with any elementsequivalent thereto, including those explicitly, necessarily, orinherently disclosed herein, for the intended purposes. Further, thepresent invention can equally be applied to apparatuses and methods.

The embodiments will be explained with respect to preferred embodiments.However, the present invention is not limited to the preferredembodiments.

Some embodiments provide a method for etching a target layer on asubstrate by a dry etching process which comprises at least one etchingcycle, wherein an etching cycle comprises: (i) depositing a carbonhalide film using reactive species on the target layer on the substrate,wherein the carbon halide film and the target layer are in contact witheach other; and (ii) (1) etching the carbon halide film using a plasmaof a non-halogen hydrogen-containing etching gas without etching thetarget layer, which plasma alone does not substantially etch the targetlayer, and thereby (2) generating a hydrogen halide as etchant speciesat a boundary region of the carbon halide film and the target layer,thereby etching a portion of the target layer in the boundary region.The “reactive species” in step (i) do not substantially etch the targetlayer but form an etchant film on the target layer. In the disclosure,“substantially zero” or the like (e.g., “not substantially etch”) mayrefer to an immaterial quantity, less than a detectable quantity, aquantity that does not materially affect the target or intendedproperties, or a quantity recognized by a skilled artisan as nearlyzero, such as that less than 10%, less than 5%, less than 1%, or anyranges thereof relative to the total or the referenced value in someembodiments.

In some embodiments, the target layer is constituted by silicon or metaloxide, wherein the metal may be Ti, W, Ta, etc., such as SiO₂, metaloxide (e.g., TiO₂), or a metal alone which is etchable by a hydrogenhalide, etc. When an etchant gas contains Cl or Br, the target layer maybe constituted by Al₂O₃, AN, GaAs, GaN, GaP, InP, etc. In someembodiments, the target layer may be constituted by polyvinyl chloridewhen an etchant gas contains Cl. A skilled artisan can determine apossible combination of an etchant gas and a target layer, based onroutine experimentation as necessary. In some embodiments, the targetlayer is formed in trenches or vias including side walls and bottomsurfaces, and/or flat surfaces, by plasma-enhanced CVD, thermal CVD,cyclic CVD, plasma-enhanced ALD, thermal ALD, radical-enhanced ALD, orany other thin film deposition methods. Typically, the thickness of thetarget layer is in a range of about 50 nm to about 500 nm (a desiredfilm thickness can be selected as deemed appropriate according to theapplication and purpose of film, etc.).

In some embodiments, in step (i), the carbon halide film is deposited bya gas phase reaction wherein the reactive species are those of anetchant gas or gases constituted by a halogen and a carbon. In someembodiments, the halogen is F, Cl, or Br. In some embodiments, anysuitable etchant gases including conventional etchant gases (e.g.,discussed in the section of “Related Art”) can be used. Since theetchant gas in step (i) does not serve as a reactive etching gas whichetches directly the target layer, but serves as a gas for deposition,preferably, the etchant gas is CxFy having a double or triple bondwherein x and y are integers and x is at least 2, e.g., C₂F₂, C₂F₄,C₃F₆, C₄F₈, C₅F₈, C₅F₁₀, or any combination of the foregoing. Thesegases tend to readily form a fluoropolymer in an excited state. In step(i), the reactant gas is selected in order to deposit an etchant film,rather than etching the target layer, e.g., no oxygen-containing gas isused since an oxygen plasma generates active etching species from theetchant gas for etching a silicon oxide film or the like. In someembodiments, the reactant gas is a noble gas such as Ar and He. In someembodiments, by selecting a suitable reactant gas and other depositionconditions, an etchant gas that is usually used for etching an siliconoxide film, such as CF₄, C₂F₆, C₃F₈, C₄F₁₀, etc. can be used.

In some embodiments, an etchant gas other than that containing fluorinemay be used for depositing an etchant film in step (i). For example, analkyl halide such as C₂H₃Cl can be used. Further, SF₆, HCl, HBr, etc.can be used in combination with a hydrocarbon such as CH₄.

In some embodiments, an etchant film may be deposited by a surfacereaction such as atomic layer deposition (ALD), wherein the etchant gaschemisorbs onto a surface of the target layer, followed by exposing thesurface to reactive species of a reactant gas.

In some embodiments, step (i) uses a gas phase reaction which isplasma-enhanced CVD. In some embodiments, the plasma-enhanced CVDcomprises: (a) continuously feeding a noble gas to a reaction spacewherein the substrate is placed; (b) continuously feeding a carbonhalide gas to the reaction space; and (c) after elapse of a presetduration of steps (a) and (b) without excitation of the noble gas andthe carbon halide gas, applying RF power to the reaction space todeposit the carbon halide film on the target layer, wherein no oxidizinggas is fed to the reaction space throughout steps (i) to (iii). In theabove, the term “continuously” refers to without interruption in space(e.g., uninterrupted supply over the substrate), without interruption inflow (e.g., uninterrupted inflow), and/or at a constant rate (the termneed not satisfy all of the foregoing simultaneously), depending on theembodiment. In some embodiments, “continuous” flow has a constant flowrate (alternatively, even though the flow is “continuous”, its flow ratemay be changed with time). In some embodiments, a duration of step (c)is shorter than the preset duration of steps (a) and (b). Since RF poweris applied to the reaction space for a short time for deposition, it isimportant to fill the reaction space fully with the carbon halide gasbefore applying RF power.

In some embodiments, step (i) continues until a thickness of the carbonhalide film falls within a range of 0.5 nm to 10 nm, preferably 1 nm to5 nm, which is near a plateau thickness (or saturation thickness),beyond which an etched quantity of the target layer in step (ii) doesnot increase even if the thickness of the carbon halide film furtherincreases.

In some embodiments, a duration of step (i) is correlated with athickness of the etched portion of the target layer until the thicknessof the etched portion of the target layer reaches a plateau while theduration of step (i) increases, and step (i) continues until thethickness of the etched portion of the target layer reaches the plateauor a point near the plateau. The mechanisms of the above are discussedearlier in this disclosure, although the mechanisms are not intended tolimit the invention.

In some embodiments, step (ii) continues until the carbon halide film issubstantially entirely etched, indicating that substantially the entireportion of the boundary region of the target layer is etched. In theabove, “substantially the entirety” or the like may refer to theentirety short by an immaterial quantity, by a detectable quantity, by aquantity that does not materially affect the target or intendedproperties, or by a quantity recognized by a skilled artisan as aninsignificant value, such as that less than 10%, less than 5%, less than1%, or any ranges thereof relative to the total or the referenced valuein some embodiments. Preferably, step (ii) continues until the carbonhalide film is completely etched, indicating that the entire portion ofthe boundary region of the target layer is completely removed. The“boundary region” of the target layer is defined as a region which isetched when the carbon halide film is completely etched. When a residueof the carbon halide film remains on the surface of the target layer,the residue may at least partially interfere with etching of the targetlayer, affecting in-plane uniformity of etched depth of the targetlayer.

In some embodiments, in step (ii), a thickness of the etched portion ofthe target layer is 0.1 nm to 2.0 nm, preferably 0.5 nm to 1.0 nm, whichis thicker than a thickness of a monolayer defined in atomic layeretching (ALE) which is less than 0.1 nm/cycle.

In some embodiments, in step (ii), the non-halogen hydrogen-containingetching gas is hydrogen or ammonia or a mixture of hydrogen or ammoniaand a rare gas. In some embodiments, the hydrogen-containing etching gascontains no oxygen. However, any suitable reactant gas can be selectedto activate the etchant film for etching the target layer by reactiveion etching. In some embodiments, a noble such as Ar and He gas, ornitrogen gas may be used as a reactant gas in combination with ahydrogen-containing gas.

In some embodiments, in step (ii), the carbon halide film is etched byreactive ion etching (RIE). The RIE may be inductively-coupled plasmaetching or capacitively-coupled plasma etching. In some embodiments, thecapacitively-coupled plasma etching comprises: (a) continuously feedinga reactant gas to a reaction space wherein the substrate is placed; and(b) after elapse of a preset duration of step (a) without excitation ofthe reactant gas, applying RF power to the reaction space to etch thecarbon halide film and the target layer.

In some embodiments, the etching cycle comprised of steps (i) and (ii)is repeated at least two times until a desired etched depth of thetarget layer is obtained. Since the etching cycle is a self-limitingetching process or saturation process, the etched depth of the targetlayer is proportional to the number of cycles performed.

In some embodiments, step (i) and step (ii) are continuously conductedin the same reaction chamber. In the above, the word “continuously”refers to at least one of the following: without breaking a vacuum,without being exposed to air, without opening a chamber, as an in-situprocess, without interruption as a step in sequence, without changingprocess conditions, and without causing chemical changes on a substratesurface between steps, depending on the embodiment. In some embodiments,an auxiliary step such as purging or other negligible step in thecontext does not count as a step, and thus, the word “continuously” doesnot exclude being intervened with the auxiliary step.

Some embodiments are explained with reference to the drawings, but arenot intended to limit the invention.

FIG. 3 illustrates schematic drawings of one cycle of an etching processaccording to an embodiment of the present invention, wherein (a)represents a deposition step, (b) represents a pre-etching step, and (c)represents an etching step. Prior to the deposition step (step (a)), asubstrate 41 (e.g., a silicon wafer or other semiconductor wafer), onwhich a target layer 42 is formed, is provided in a reaction space. Inthe deposition step, a carbon halide film (an etchant film) 43 isdeposited on the target layer 42 at a thickness no less than asaturation thickness T1, wherein the carbon halide film 43 and thetarget layer 42 are in contact with each other. The saturation thicknessT1 is defined as a thickness, a portion of the etchant film beyond whichdoes not contribute to etching of the target layer, i.e., an etchedquantity of the target layer does not increase even if the thickness ofthe etchant film further increases beyond the saturation thickness T1.In the pre-etching step (step (b)), the etchant film 43 is exposed to aplasma such as a hydrogen plasma (hydrogen radicals H*), thereby etchingthe etchant film, generating gaseous components such as hydrocarbon gasand hydrogen halide, etc. therefrom, and removing the top portion of theetchant film 43 above the saturation thickness T1 through the followingchemical reaction, for example (where the etchant film is constituted byCxFy wherein x and y are integers):

CxFy+(4x+y)H*→xCH₄ +yHF  (1)

The above reaction is a general reaction and simplified since thecomposition of fluorocarbon varies depending on the deposition method,the type of etchant gas, etc. Further, although hydrocarbons other thanCH₄ may be generated, such hydrocarbons are likely to be eventuallydecomposed to CH₄, and thus, the reaction can be represented by CH₄.Through reaction (1), by the hydrogen radicals, carbon components can beremoved from the etchant film, while generating HF components, which inturn etch the SiO₂ film.

When the etching of the etchant film 43 progresses and the etchant film43 is etched to the saturation thickness T1, gaseous components such asHF, etc. from the etchant film 43 simultaneously generate gaseouscomponents such as SiF₄, 2H₂O, etc. from a portion of the target layer42 having a depth T2 by using the HF gas at a boundary region 44 throughthe following chemical reaction, for example:

SiO₂+4HF→SiF₄+2H₂O  (2)

Through reaction (2), the SiO₂ film is etched wherein generated H₂O canfurther assist the etching reaction. This etching is chemical etching,and thus, etching selectivity can remarkably be high, e.g., between SiO₂and SiN (high etch rate against SiO₂, but substantially no etch rateagainst SiN), and also, conformal etching (uniform etching) can beperformed. These features cannot be achieved by conventional etchingmethods.

The boundary region 44 is comprised of a boundary region of the etchantfilm 43 having the saturation thickness T1 and a boundary region of thetarget layer 42 having a depth T2. The total thickness of the boundaryregion (T1+T2) may depend on the ion energy in plasma, e.g., dependingon RF power and the pressure of the reaction space. The boundary region44 may be composed of an intermediate constituted by mixed componentssuch as SiCOF. As shown in reactions (1) and (2), the pre-etching step(step (b)) is comprised of two steps, wherein the first step is etchingthe etchant film (e.g., carbon halide film) and generating etchantspecies (e.g., hydrogen halide) by hydrogen radicals, and the secondstep is etching the target layer by the etchant species. The second stepcan be performed without a plasma (without applying RF power), whereasthe first step uses a plasma. However, the first and second stepsoverlap each other, i.e., the second step of etching the target layer bythe etchant species starts before the end of the first step.

In the etching step (step (c)), the boundary region or intermediatelayer 44 is removed as gaseous components, wherein the target layer 42is etched by the depth T2 to obtain an etched target layer 45. It shouldbe noted that although steps (b) and (c) are separately shown for aneasy understanding of the principle of the steps, these steps ratherconcurrently occur. Since the plasma alone does not substantially etchthe target layer, the etching of the target layer 45 stops when theboundary region 44 is removed.

In some embodiments, the process may be performed as illustrated in FIG.4 which shows a flow chart of an atomic layer etching (ALE). In step a,a CF film is deposited by plasma-enhanced or thermal CVD or any suitablemethod which can preferably perform conformal deposition (e.g.,hexafluoropropylene oxide (HFPO) thermal deposition). After step b(purging the reaction space), in step c, the CF film is exposed to ahydrogen plasma, thereby generating HF at a surface of the target layer(e.g., SiO₂). In this process, the presence of hydrogen radicals may besufficient or additionally, ion bombardment may be necessary in someembodiments. After step c, the reaction space is purged in step d. Stepsa to d constitute one cycle of ALE, which may be repeated multiple timesuntil a desired thickness of the target layer is etched. Since theetchant film (CF film) is exposed to a hydrogen plasma and etched, andthe target layer is not exposed to the hydrogen plasma, less orsubstantially no plasma damage is caused to the target layer.

In some embodiments, the process sequence may be set as illustrated inFIG. 2. FIG. 2 shows a schematic process sequence of plasma-assistedcyclic etching in one cycle according to an embodiment of the presentinvention wherein a step illustrated in the sequence represents an ONstate whereas no step illustrated in the sequence represents an OFFstate, and the length of each ON and OFF states does not representduration of each process. In this embodiment, one etching cyclecomprises a deposition step (plasma-enhanced CVD in this embodiment) andan etching step. The deposition step comprises “Step 1”, and the etchingstep comprises “Purge 1”, “Step 2”, “Step 3”, and “Purge 2”.

In “Step 1” of the deposition step which is a plasma-enhanced CVD step,a dilution gas such as Ar and/or He is continuously fed to a reactionspace wherein the substrate is placed, while a carbon halide gas iscontinuously fed to the reaction space, while continuously applying RFpower to the reaction space without feeding a hydrogen gas. Ahalogen-containing film (e.g., a carbon halide film) having a desiredthickness (e.g., 0.5 nm to 10 nm, typically about 2 nm) is deposited asan etchant film on a target layer in “Step 1” of the deposition step.Step 1 is a step of deposition of the etchant film (extending in adirection perpendicular to a thickness direction without pinholes andentirely covering a concerned area of the target layer), which step ismore than a step of providing reactive sites on the target layer. Thedeposition step may be any suitable deposition step including those ofthermal CVD, PVD, PEALD, etc., and a skilled artisan in the art canreadily modify “Step 1” accordingly.

After “Step 1”, “Purge 1” of the etching step starts, wherein a hydrogengas (etching gas) is continuously fed to the reaction space withoutfeeding the dilution gas and the carbon halide gas and without applyingRF power. The etching gas may be a gas containing hydrogen such asammonia or a mixture of a hydrogen-containing gas and a rare gas (e.g.,Ar, He). “Purge 1” is for removing non-reacted products, unwantedby-products, and any remaining deposition gas(es) and for stabilizingthe reaction space and getting ready for etching. After elapse of apreset duration of “Purge 1”, in “Step 2”, RF power is applied to thereaction space to etch the halogen-containing film (etchant film, e.g.,carbon halide film) and the target layer while feeding the hydrogen gas(etching gas) without feeding the halogen-containing gas to generatehydrogen radicals which in turn generates etchant species such as ahydrogen halide as a result of reaction with the etchant film such ascarbon halide. The etchant species such as a hydrogen halide chemicallyetches the target layer surface in contact with the etchant species atthe boundary region between the target layer and the halogen-containingfilm. In “Step 2”, the chemical etching of the target layer begins andprogresses as the etchant species such as a hydrogen halide isgenerated, and in “Step 3” where the hydrogen gas flow and theapplication of RF power are stopped, since some of the etchant speciesremains, the residual etchant species continues etching the target layersurface until the residual etchant species is consumed or “Purge 2”begins. Any suitable means for generating hydrogen radicals can be usedin “Stet 2”. The duration of “Step 2” can be determined according to thethickness of the halogen-containing film, etc. In “Step 2”, thehalogen-containing film is etched or removed in its entirety. Theduration of “Step 3” can be determined according to how much residualhydrogen halide is left at the end of “Step 2” and how much more thetarget layer surface needs to be etched, etc. In “Purge 3”, the reactionspace is purged to remove non-reacted products, unwanted by-products,and any remaining etchant species by feeding the dilution gas. Thisetching cycle can be repeated until a desired depth of the target layeris etched.

In “Step 2”, RF power is used for generating a hydrogen plasmacontaining hydrogen radicals, and thus, alternatively, by using a remoteplasma unit, for example, a hydrogen plasma containing hydrogen radicalscan also be generated. In some embodiments, RF power is applied inpulses.

In some embodiments, the etching cycle may be conducted under theconditions shown in Table 1 below.

TABLE 1 (numbers are approximate) Conditions for deposition (PECVD as anexample) Substrate temperature 0 to 200° C. (preferably 20 to 100° C.)Pressure 0.1 to 10000 Pa (preferably 1 to 1000 Pa) Noble gas (as acarrier gas and/or Ar, He dilution gas) Flow rate of carrier gas 1 to5000 sccm (preferably 1 to 2000 sccm) (continuous) Flow rate of dilutiongas 10 to 10000 sccm (preferably 50 to 5000 sccm) (continuous) Flow rateof etchant gas 1 to 1000 sccm (preferably 10 to 100 sccm); Correspondingto the flow rate of carrier gas when the etchant is vaporized using aheated bottle RF power for a 300-mm wafer 10 to 1000 W (preferably 50 to200 W); 0.1 to 100 MHz (preferably 0.4 to 60 MHz) Duration of “RF”(Step 1) 0.1 to 10 sec. (preferably 1 to 5 sec.) Growth rate per cycle({acute over (Å)}/cycle) 5 to 100 (preferably 40 to 80) Film thickness({acute over (Å)}) 5 to 100 (preferably 10 to 50) Conditions for etchingSubstrate temperature 0 to 200° C. (preferably 20 to 100° C.) Pressure0.1 to 10000 Pa (preferably 1 to 1000 Pa) Etching gas H₂, NH₃, H₂ +Ar/He, or NH₃ + Ar/He, Flow rate of etching gas 10 to 10000 sccm(preferably 50 to 5000 sccm) for hydrogen-containing gas; 10 to 10000sccm (preferably 50 to 5000 sccm) for rare gas RF power for a 300-mmwafer 10 to 1000 W (preferably 50 to 200 W); 0.1 to 100 MHz (preferably0.4 to 60 MHz) Duration of “Purge 1” 1 to 60 sec. (preferably to 10sec., depending on chamber structure) Duration of “Step 2” 1 to 120 sec.(preferably 10 to 30 sec.) Duration of “Step 3” 1 to 60 sec. (preferablyto 10 sec., depending on chamber structure) Duration of “Purge 2” 1 to60 sec. (preferably to 10 sec., depending on chamber structure) Etchingrate per cycle ({acute over (Å)}/cycle) 1 to 50 (preferably 2 to 10)Etched thickness ({acute over (Å)}) 1 to 1000 (preferably 10 to 100)

In the sequence illustrated in FIG. 2, the halogen-containing gas may besupplied using a carrier gas which can be continuously supplied to thereaction space, particularly when the halogen-containing gas is avaporized gas of liquid material such as C₅F₈, C₅F₁₀, etc. This can beaccomplished using a flow-pass switching (FPS) system (e.g., the systemdisclosed in U.S. patent application Ser. No. 14/829,565, filed Aug. 18,2015, the disclosure of which is incorporated by reference in itsentirety) wherein a carrier gas line is provided with a detour linehaving a precursor reservoir (bottle), and the main line and the detourline are switched, wherein when only a carrier gas is intended to be fedto a reaction chamber, the detour line is closed, whereas when both thecarrier gas and a precursor gas are intended to be fed to the reactionchamber, the main line is closed and the carrier gas flows through thedetour line and flows out from the bottle together with the precursorgas. In this way, the carrier gas can continuously flow into thereaction chamber, and can carry the precursor gas in pulses by switchingthe main line and the detour line.

In some embodiments, the target layer is constituted by Si alone, ametal alone, an oxide of the foregoing, or a carbonate of the foregoing,and accordingly, a suitable etchant gas (halogen-containing gas) can beselected which can form an etchant film (halogen-containing film) whichin turn generates etchant species when being etched by hydrogenradicals. For example, when the target layer is constituted by SiO₂, theetchant gas suitably contains F as a halogen, whereas when the targetlayer is constituted by Si, the etchant gas suitably contains Cl as ahalogen. A skilled artisan in the art can readily select a suitableetchant gas for the target layer in view of the present disclosure andthrough routine experimentation. Table 2 below shows examples. Since theetching process by the etchant species has high selectivity, it ispossible to selectively etch only silicon oxide while keeping siliconnitride, for example. The etchant film may be constituted by a carbonhalide which can be decomposed to generate etchant species such as ahydrogen halide by hydrogen radicals. The hydrogen radicals can beproduced from an etchant gas containing hydrogen or a mixture of ahydrogen-containing gas and a rare gas.

TABLE 2 (Suitable combination) Target layer Etchant film Si, Al (a metalsoluble in HCl) CCl film (by PVC, etc.) Ti, W (a metal soluble in HCl)CF film Silicon oxide CF film Silicon nitride Not etchable Metal oxide(e.g., TiO) CF film

The process cycle can be performed using any suitable apparatusincluding an apparatus illustrated in FIG. 1, for example. FIG. 1 is aschematic view of a plasma-assisted cyclic etching apparatus, desirablyin conjunction with controls programmed to conduct the sequencesdescribed below, usable in some embodiments of the present invention. Inthis figure, by providing a pair of electrically conductive flat-plateelectrodes 4, 2 in parallel and facing each other in the interior 11(reaction zone) of a reaction chamber 3, applying HRF power (13.56 MHzor 27 MHz) 20 to one side, and electrically grounding the other side 12,a plasma is excited between the electrodes. A temperature regulator isprovided in a lower stage 2 (the lower electrode), and a temperature ofa substrate 1 placed thereon is kept constant at a given temperature.The upper electrode 4 serves as a shower plate as well, and noble gas,reactant gas, and etchant gas are introduced into the reaction chamber 3through gas lines 21, 22, and 23, respectively, and through the showerplate 4. Additionally, in the reaction chamber 3, a circular duct 13with an exhaust line 7 is provided, through which gas in the interior 11of the reaction chamber 3 is exhausted. Additionally, a dilution gas isintroduced into the reaction chamber 3 through a gas line 23. Further, atransfer chamber 5 disposed below the reaction chamber 3 is providedwith a seal gas line 24 to introduce seal gas into the interior 11 ofthe reaction chamber 3 via the interior 16 (transfer zone) of thetransfer chamber 5 wherein a separation plate 14 for separating thereaction zone and the transfer zone is provided (a gate valve throughwhich a wafer is transferred into or from the transfer chamber 5 isomitted from this figure). The transfer chamber is also provided with anexhaust line 6. In some embodiments, the deposition of multi-elementfilm and surface treatment are performed in the same reaction space, sothat all the steps can continuously be conducted without exposing thesubstrate to air or other oxygen-containing atmosphere. In someembodiments, a remote plasma unit can be used for exciting a gas.

In some embodiments, a dual chamber reactor (two sections orcompartments for processing wafers disposed closely to each other) canbe used, wherein a reactant gas and a noble gas can be supplied througha shared line whereas a precursor gas is supplied through unsharedlines. In some embodiments, the deposition step can be performed usingan apparatus different from that for the etching step.

A skilled artisan will appreciate that the apparatus includes one ormore controller(s) (not shown) programmed or otherwise configured tocause the deposition and reactor cleaning processes described elsewhereherein to be conducted. The controller(s) are communicated with thevarious power sources, heating systems, pumps, robotics, and gas flowcontrollers or valves of the reactor, as will be appreciated by theskilled artisan.

The present invention is further explained with reference to workingexamples below. However, the examples are not intended to limit thepresent invention. In the examples where conditions and/or structuresare not specified, the skilled artisan in the art can readily providesuch conditions and/or structures, in view of the present disclosure, asa matter of routine experimentation. Also, the numbers applied in thespecific examples can be modified by a range of at least ±50% in someembodiments, and the numbers are approximate.

EXAMPLES Example 1 (Prophetic Example)

A silicon oxide film, silicon nitride film, and silicon carbide film areformed at a thickness of about 20 nm by PEALD or PECVD as a target layeron 300-mm substrates, respectively. Deposition of an etchant film (CFfilm) and etching of each target layer are conducted under theconditions shown in Table 3 below using the plasma-assisted etchingapparatus illustrated in FIG. 1. The sequence used in each etching cycleis shown in FIG. 2.

TABLE 3 (numbers are approximate) Conditions for deposition (PECVD)Substrate temperature Room temperature Pressure 2.0 Pa Deposition gasC₂F₆ Noble gas (as a dilution gas) Ar Flow rate of deposition gas 10sccm (etchant gas) (continuous) Flow rate of dilution gas 2000 sccm(continuous) RF power for a 300-mm wafer 100 W; 13.56 MHz (or 27 MHz, 6MHz, or microwaves) Duration of “RF” (Step 1) 2.0 sec. Distance betweenelectrodes 30 mm Growth rate per cycle (nm/cycle) 0.7 nm/cyc Filmthickness (nm) 2.0 nm Conditions for etching Substrate temperature Sameas in deposition Pressure Same as in deposition Etching gas H₂ + Ar Flowrate of etching gas H₂: 100 sccm; Ar: 1000 sccm RF power for a 300-mmwafer Same as in deposition Duration of “Purge 1”  60 sec. Duration of“Step 2”  60 sec. Duration of “Step 3”   5 sec. Duration of “Purge 2” 60 sec.

The etching cycle comprising the deposition step and the etching step isrepeated 3 and 6 times for each substrate. After the etching cycles, thethickness of each target layer is measured by ellipsometry. As a result,the etching rate per cycle (EPC) of the SiO₂ film is about 0.7 nm/cycle,whereas those of the SiN film and the SiC film are nearly zero, i.e.,substantially no reduction of the film thickness.

It will be understood by those of skill in the art that numerous andvarious modifications can be made without departing from the spirit ofthe present invention. Therefore, it should be clearly understood thatthe forms of the present invention are illustrative only and are notintended to limit the scope of the present invention.

We/I claim:
 1. A method for etching a target layer on a substrate by adry etching process which comprises at least one etching cycle, whereinan etching cycle comprises: (i) depositing a carbon halide film usingreactive species on the target layer on the substrate, wherein thecarbon halide film and the target layer are in contact with each other;and (ii) (1) etching the carbon halide film using a plasma of anon-halogen hydrogen-containing etching gas without etching the targetlayer, which plasma alone does not substantially etch the target layer,and thereby (2) generating a hydrogen halide as etchant species at aboundary region of the carbon halide film and the target layer, therebyetching a portion of the target layer in the boundary region.
 2. Themethod according to claim 1, wherein step (ii) continues until thecarbon halide film is substantially entirely etched, indicating thatsubstantially the entire portion of the boundary region of the targetlayer is etched.
 3. The method according to claim 1, wherein a durationof step (i) is correlated with a thickness of the etched portion of thetarget layer until the thickness of the etched portion of the targetlayer reaches a plateau while the duration of step (i) increases, andstep (i) continues until the thickness of the etched portion of thetarget layer reaches the plateau or a point near the plateau.
 4. Themethod according to claim 1, wherein the etching cycle is repeated atleast two times.
 5. The method according to claim 1, wherein step (i)continues until a thickness of the carbon halide film falls within arange of 0.5 nm to 10 nm.
 6. The method according to claim 1, wherein instep (ii), a thickness of the etched portion of the target layer is 0.1nm to 2.0 nm.
 7. The method according to claim 1, wherein in step (i),the carbon halide film is deposited by a gas phase reaction wherein thereactive species are those of an etchant gas or gases constituted by ahalogen and a carbon.
 8. The method according to claim 7, wherein thehalogen is F or Cl.
 9. The method according to claim 8, wherein thehalogen is F, and the etchant species is HF.
 10. The method according toclaim 8, wherein the etchant gas is CxFy having a double or triple bondwherein x and y are integers and x is at least
 2. 11. The methodaccording to claim 7, wherein the gas phase reaction is plasma-enhancedCVD.
 12. The method according to claim 11, wherein the plasma-enhancedCVD comprises: (a) continuously feeding a noble gas to a reaction spacewherein the substrate is placed; (b) continuously feeding a carbonhalide gas to the reaction space; and (c) after elapse of a presetduration of steps (a) and (b) without excitation of the noble gas andthe carbon halide gas, applying RF power to the reaction space todeposit the carbon halide film on the target layer, wherein no oxidizinggas is fed to the reaction space throughout steps (a) trough (c). 13.The method according to claim 12, wherein a duration of step (c) isshorter than the preset duration of steps (a) and (b).
 14. The methodaccording to claim 1, wherein in step (ii), the non-halogenhydrogen-containing etching gas is hydrogen or ammonia or a mixture ofhydrogen or ammonia and a rare gas and contains no oxygen.
 15. Themethod according to claim 1, wherein in step (ii), the carbon halidefilm is etched by reactive ion etching (RIE).
 16. The method accordingto claim 15, wherein the RIE is capacitively-coupled plasma etching. 17.The method according to claim 16, wherein the capacitively-coupledplasma etching comprises: (a) continuously feeding a reactant gas to areaction space wherein the substrate is placed; and (b) after elapse ofa preset duration of step (a) without excitation of the reactant gas,applying RF power to the reaction space to etch the carbon halide filmand the target layer.
 18. The method according to claim 1, wherein thetarget layer is constituted by SiO₂, metal oxide, or a metal, which isetchable by a hydrogen halide.
 19. The method according to claim 1,wherein step (i) and step (ii) are continuously conducted in the samereaction chamber.